Machines are used in many areas which have components moved automatically by a drive device or which are moved themselves. If persons are standing in the movement zone of these components, the persons can be potentially at risk from the movement of the components. For the protection of people, protective apparatuses can therefore be arranged at the moved component at such machines which allow a monitoring of a protected zone moved along with the component. If the presence of a person is detected in the moved along protected zone, the movement of the component can be interrupted.
An example for such a machine is a bending press which is shown schematically in part in FIG. 1 and which has a bending tool 10 driven by a drive device not shown in FIG. 1 and having, for example, a lower side formed in a V shape and a stationary lower tool 12 whose upper side can, for example, have a V groove corresponding to the shape of the lower side of the bending tool 10. For the shaping of a workpiece 14, it is placed by an operator onto the lower tool 12. To achieve a short cycle time, the bending tool 10 is moved from a starting position at high speed along a direction of movement B toward the workpiece 14, but is then braked again such that it impacts the workpiece 14 at a lower speed suitable for the bending process and presses said workpiece into the lower tool 12. The slowing down of the movement of the bending tool 10 can not take place instantaneously due, among other things, to its inertia. The path which is necessary to brake the bending tool 10 down from the fast speed to a standstill is also termed the trailing path N.
For the protection of a person who places workpieces 14 into the bending press, a light transmitter 20 and a light receiver 22 are attached to two holding arms 16 and 18 and together form a one-way light barrier. The light transmitter 20, which has a light source 24 and a focusing optical transmitting system 26, radiates a substantially parallel transmitted light beam 27 at a distance to the trailing path N parallel to the lower side of the bending tool 10 onto the light receiver 22 which has a photo-detection element with evaluation electronics 28 and an optical reception system 30 for the focusing of the received light ray onto the photo-detection element 28 with the evaluation electronics. The evaluation electronics are made such that, when the light path between the light transmitter 20 and the light receiver 22 is interrupted, a signal can be emitted to the drive device so that it brakes the movement of the bending tool 10 as fast as possible. The parallel transmitted light beam emitted by the light transmitter 20 therefore defines a protected zone which prevents injury to a person thanks to its distance from the lower side of the bending tool 10 corresponding to the size of the trailing path N.
In practice, however, vibrations of the holding arms 16 and 18 occur due to the movement of the bending tool 10, whereby the light transmitter 20, and thus also the parallel transmitted light beam 27 emitted by the light transmitter, is, for example, tilted relative to the light transmitter 20 (illustrated in FIG. 1 by short-dotted lines). Due to the long light path, even small angles of tilt can result in large movements of the transmitted light beam 27 in the plane of the light receiver 22, and in particular in a migration from a reception surface of the light receiver 22. The transmitted light beam 27 can thereby no longer be received by the light transmitter 22 even without the presence of a body part of a person between the bending tool 10 and the workpiece 14. The evaluation optics will therefore detect an interruption of the light path such that the bending press is stopped. This increases the downtimes of the bending press in an unnecessary manner.
To reduce the downtime, a reception surface of the light receiver 22 can alternatively be enlarged such that the transmitted light ray 27 of the light transmitter 20 does not migrate so easily from the light receiver 22 on vibrations. However, the case can then occur that the presence of a body part of a person between the bending tool 10 and the workpiece 14—in FIG. 1, for example, close to the light receiver 22—cannot be detected although it is at a distance beneath the bending tool 10 inside the trailing path N. This then means a reduction in labor safety.
Bending presses secured in this manner therefore have the disadvantage that they either conceal increased safety risks for the operators or have undesirably high downtimes.